In the handling of radioactive substances as well as in operating accelerators, x-ray equipment and sources of stray radiation, radiation protection areas are to be established to provide protection for persons in which for each case specific protection rules apply. This requires a monitoring, i.e. a continuous measuring of the radiation. For this, so-called dosimeters are used.
For radiation monitoring, particularly with accelerators, aside from the measuring of the neutron radiation level, the measuring of photon radiation is also necessary. With the implementation of the new radiation protection regulations and x-ray regulations, new measurements for the area dosage and person dosage are to be used. The dosage rate used until the present of “photon equivalent dose Hx” has been replaced with the dosage rate “ambient equivalent dose H*(10)” for penetrating radiation.
The dosage rate used until the present is based on the dosage generated through radiation in the absence of air; the new dosage rate is defined through the dosage which exists through an identical radiation in 10 mm depth of a standardized test body (ICRU-ball; ICRU: International Commission on Radiation, Units and Measurements). The definition is as follows: The ambient equivalent dose H*(10) at a point of interest in the actual radiation field is the equivalent dose generated in the respective established and expanded radiation field at a 10 mm depth in the ICRU-ball at the radius vector in the opposite direction of the radiation angle of incidence. Both active, i.e. electronic area dosimeters and passive area dosimeters exist. Known active monitors include, for example, scintillation dosimeters, Geiger-Müller counters, proportional counters and ionization chambers.
A dosimeter for low energy x-rays and gamma rays is known of from the DE 697 11 199 T2, whereby, among other things, the ambient equivalent dose H* can be measured. This dosimeter uses a silicon based photodiode and a second diamond based detector, whereby the test signals are electronically processed by current preamplifiers and analog digital converters.
With active monitors such as, for example the type FHZ 600A (distributed by Thermo Electron, Erlangen) the levels of the gamma radiation produced, for example in the experiment hall EH as well as in the area of the experiments conducted on the synchrotron of the Association for Heavy Ion Research mbh [translation of: Gesellschaft für Schwerionenforschung mbh] are recorded. Active monitors have the disadvantage that they are complicated and expensive, and require a power source or regular replacement of the batteries. Furthermore, they may be overloaded by short powerful radiation pulses, which may occur, in particular, with pulse driven accelerators, which may result in a corruption of the monitoring results. As a result, passive dosimeters are used, ideally, for the measurement of pulsed x-rays and gamma rays.
A process for measurement of radiation dosage which is for the most part less than 45 keV is known of from the DE 1 489 922. With this process two phosphate glass measurement elements with differing casings are used to record a measurement through subtraction which is not affected by energy. Furthermore, radiation between 40-80 keV is not taken into account as these values are cancelled out through the subtraction process.
Passive area dosimeters typically contain a passive detection element, whereby said elements absorb and store the incident radiation in a physical process without the necessity of electrical current. Thermoluminescent detectors (TLD) are a typical example of this. Thermoluminescent detectors contain, for example, lithium fluoride crystals of the isotope 6LiF or 7LiF, whereby although the response characteristics of 6LiF and 7LiF for neutron radiation are different, they have the same response characteristics for photon radiation. Thermoluminescent detectors of this sort are available, for example, from the company Thermo Electron GmbH. Four 7LiF crystals may be, for example, applied to a thermoluminescent detector card. The irradiated detector cards are evaluated in a machine. In a heating process, the light emitted from the TLD is detected using photomultipliers, and a so-called glow curve is recorded. The dosage is determined by means of the measured glow curves.
A dosimeter with a dosimeter card having the aforementioned lithium fluoride crystals is known of from the DE 39 03 113 A1. In this case however, it is not dealing with an area dosimeter, but rather a personal dosimeter and thereby mainly with the design of the detector card. This is to be constructed in the shape of a square, in order to enlarge the distance between the crystals.
H*(10) area dosimeters are known of from Siebersdorf Research which contain an aluminum dosimeter card with four lithium fluoride chips. The dosimeter cards are laminated in a composite plastics film to protect said from contamination and are inserted from above through an opening in a plastic cylinder.
Subsequently, the dosimeter is sealed with a powder coated aluminum protection cap, to be placed or hung at the measuring location, for measurement during a three-month measuring period.
The disadvantage with these area dosimeters is, firstly, that the response capacity of the thermoluminescent detectors displays a strong dependence on energy. The measuring rate is predefined through legal regulations, such as the German radiation protection regulation (see: regulation for the protection from damage by ionizing radiation (radiation protection regulation—Strahlenschutzverordnung—StrlSchV) of Jul. 20, 2001), with the aforementioned measuring rate H*(10), which is to be measured by an ambient dose equivalent dosimeter.
The energy dependency for pure thermoluminescent crystals however is only similar to the measurement rate H*(10) within an energy range of circa 100 keV-1 MeV. Despite the plastic cylinder, the H*(10) area dosimeter of Seibersdorf Research is also provided with an energy utility range of only 30 keV-1.3 MeV. Particularly problematic for many areas of application is the faulty precision in the range between 10 keV and 30 keV. In general, a measurement range of 10 keV-3 MeV, or preferably, up to 10 MeV, is desirable.
Aside from this, it is questionable whether these area dosimeters have a sufficient measurement precision over the full range of 360°. It is not certain, for example, that an area dosimeter which has sufficient measurement precision at an angle of 0° for a specific energy interval also has the same precision, for example, at an angle of 75°.
Overall, there is a need for improvement regarding the existing area dosimeters.